302 THE LIAS AMMONITES. 



three digitations ; the auxiliary saddles have small festoons, and the auxiliary lobe 

 terminates in two points. The septa are placed so wide apart that we often find three 

 ribs intervening between two septa. The radial line, in parting from the extremity of 

 the dorsal lobe, cuts the points of the principal lateral lobe, but passes at some distance 

 from the lateral lobe and the auxiliaries. 



Affinities and Differences. — The compressed sides, prominent keel, and absence of 

 lateral sulci, separate this form from other Arietites with which it is grouped, and with 

 which it has many affinities both in the structure of the lobe-line and in the lobes and 

 saddles themselves. It has numerous affinities with the genus Hmyoceras in the form 

 and structure of the shell. 



Locality and Stratigraphical Position. — General Portlock found his type shell in the 

 Lias at Larne, and others at Portrush and Ballintoy. I have found it in the lower part 

 of the Armatus-hed at the base of the Jamesoni-zone in Lias Clays near Cheltenham, 

 associated with Aegoceras densinodum and Amaltheus oxgnotus. 



In France it is very rare. The type specimen figured by d'Orbigny was collected in the 

 Lias of Gros-Bois, Cote d'Or ; Dumortier records it from St. Fortunat, Limonest, Rhone ; 

 Jambles, Saone-et-Loire ; Nolay, Cote-d'Or, and other localities. Professor Von Hauer's 

 specimen, figured in his ' Nordostlichen Alpen ' was found in the Lias of the Alps ; 

 at Neustiftgraben, Steinbauer, Adneth, Weidachlahne, in Ammergau. D. Stur collected 

 numerous examples of this species, two to three inches in diameter, in the Carpathians 

 at Modern, at Tureczka, and Arva. 



Schafhautl found it in the Bavarian Alps in several localities ; Studer in Switzerland ; 

 and Savi and Meneghini in the Apennines at Castlenuovo in Garfagnana. 



Arietites impendens, Young and Bird. PI. XXII a, figs. 1 — 5. 



Ammonites impendens, Young and Bird. Geol. Surv. Yorkshire Coast, p. 266, 1828. 



— — Simpson. Monogr. of the Ammonites, p. 52, 1843. 



— Fowleri, Buckman. Geol. of Cheltenham, p. 104, pi. xii, fig. 7, 1845. 



— impendens, Simpson. Fossils of the Yorkshire Lias, p. 96, 1855. 



— eadiatus, Simpson. Ibid., p. 88, 1855. 



Arietites impendens, Tate and Blake. Yorkshire Lias, p. 290, pi. vi, fig. 7, 1876. 



Diagnosis. — Shell discoidal, depressed, and highly carinate; whorls deep ; sides con- 

 vex and bevelled towards the margin, nearly two thirds involute ; when goung, sides costate, 

 with thirty-six straight acute ribs, which bend sharply inwards toward the margin ; adult, 

 sides smooth or undulated with flat folds, which overhang the edge of the umbilical 

 suture. Siphonal area narrow, with a very prominent acute keel, and two wide, deep 

 lateral furrows ; aperture narrow, compressed, or sagittate. 



Dimensions (go/ing ; costateform, of specimen figured, PI. XXII a, fig. 4). — Diameter 

 40 millimetres; ditto of the umbilicus 12 millimetres. 



